1. Field of the Invention
The present invention relates to an optical fibre temperature sensor and to a method for determining temperature along an optical fibre.
2. Technical Background of the Invention
It relates in particular to an optical fibre temperature sensor of Raman type for the distributed monitoring of temperature along medium/high voltage electric cables. The sensor is however adaptable to other applications such as fire prevention monitoring within boreholes/tunnels, monitoring of gas/oil pipelines and for all those industrial applications requiring constant continuous temperature control along medium/long distances of interest.
Numerous patents are present on the subject of optical fibre sensors for distributed temperature measurement, these discussing the filtration/separation of the different spectral components used in temperature measurement.
To provide a reliable measurement of temperature distribution along the fibre sensor an appropriate normalization of the temperature curve must be carried out, i.e. the measurement must be normalized with reference to possible mechanical losses in the fibre sensor.
The compensation techniques typically used comprise analysis of the Stokes-anti Stokes spectral components; or analysis of the Rayleigh-anti Stokes spectral components
For example, the spectral component of Rayleigh scattering is analyzed and used as reference for temperature distribution compensation. In this respect, analysis of the Rayleigh signal component enables attenuation distribution along the sensor fibre to be determined. Knowing this pattern it is possible, by simultaneously analyzing temperature distribution and attenuation distribution, to discriminate the effects of temperature and those related to possible mechanical stresses or micro-bending phenomena induced by the sensor fibre. In this manner, temperature evaluation along the sensor fibre is more accurate and free of measurement errors.
If measurement accuracy is to be improved, several sources with different probe wavelengths could be used. In this manner, by using a suitable filtration system, the attenuation distribution along the fibre can be obtained for each source used.
This enables a more accurate reconstruction of attenuation distribution based on the probe wavelength and hence more effective compensation for losses due to mechanical stresses or micro-bending phenomena.
The Rayleigh spectral component can also be filtered if a compensation technique is to be used which evaluates the ratio, in terms of power, between the Raman anti-Stokes and Raman Stokes spectral components. This technique typically enables a clear and precise evaluation of temperature distribution but does not normally enable those effects related to temperature to be effectively discriminated from those due to losses of a mechanical nature or to reflection phenomena (splicing or connectors).
In both cases those components not used for normalization purposes are filtered out by suitable devices and not used for other purposes.
An object of the present invention is to provide an optical fibre temperature sensor and a method for determining temperature along an optical fibre able to obviate the drawbacks of the known art.
Another object is to a system which is of low cost while still providing high measurement performance.
These and further objects are attained, according to the present invention, by an optical fibre temperature sensor comprising: an optical pulse generator; an optical fibre into which said optical pulses are fed; an optical receiver to receive said optical pulses reflected by said optical fibre and to convert them into an electrical signal; a processor which receives said electrical signal and determines the temperature along said optical fibre; said optical receiver comprising a first filter and a second filter to filter said optical pulse reflected by said optical fibre, characterised in that said first and said second filters filter two adjacent portions of anti-Stokes optical signals or Stokes optical signals.